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I'm doing an acetate animation effect: Text on 'transparent plastic' lays down on a white surface, and as the text lays down its shadow, from a sun lamp, plays on the white surface.

All achieved. A subdivided plane, cloth sim, UV text, the usual diff/transparency/factor node set up.

The problem is I want there to be no residual shadow once the text has hit the white base plane. And the cloth sim won't let me hit the base plane, due to the cloth sim tolerances. It floats just above the base plane.

So I'm looking for a way to 'fade' this thin drop-shadow away in the compositor.

I can't change the world surface node colour (which gives the shadow colour) from black to white: that affects the ambient light of the entire scene. I can't raise or lower the sun lamp strength for the same reason.

I guess I'm looking for more info on the shadow pass... I'm not sure how to use it in this example. I'm looking for a parameter that allows me to fade out the lamp's ability to cast a shadow... or to fade out the ability of the base plane to receive a shadow. Any tips?

Blend file: https://dl.dropboxusercontent.com/u/13900881/flop2.blend

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This affects the object casting the sadow not the lamp.

A slightly simpler solution exists: you can set the object you want not to cast shadows not to interact with shadow rays. You can then render your animation with and without the shadow. Finally you mix the two animations to control the fading.

The parameters are in Object > Ray Visibility > Shadow. See this qustion for more info about this:

How to make a cycles light emisson object invisible to the camera?

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  • $\begingroup$ I understand the solution, but this seems like a thing that can be solved with a node setup. This simple solution just transfers the problem to an editor rather than the compositor. $\endgroup$
    – OroNZ
    Commented Feb 14, 2016 at 22:32
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OK, I think I got it!

Shadow fade

This is all on the object casting the shadow.

The lower section is just the usual "make a transparent PNG UV" set up.

The Light path and Multiply Math node target the shadow rays cast by the object. With the lower Value = 0, full shadow. Value = 1 (or higher), no shadow.

This value feeds the Mix Shader on the degree of transparency of the shadow rays only.

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There is a cast shadow tick in lamp section. You can insert key-frame to it and make shadow disapear in animation:

enter image description here

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  • $\begingroup$ Should the two be merged? $\endgroup$
    – Mörkö
    Commented Feb 14, 2016 at 10:48
  • $\begingroup$ As above, I understand (and have tried) the solution here, but two dodgy things happen 1) a 'black flash' frame occurs during the instantaneous transition from shadow to non-shadow occurs, and 2) even if I cut out the black frame the drop-shadow instantly disappears... a very jarring effect. I'm looking for a compositor/material node solution. $\endgroup$
    – OroNZ
    Commented Feb 14, 2016 at 23:14
  • $\begingroup$ This would turn the shadow off an on but will not allow for a gradual transition. $\endgroup$
    – user1853
    Commented Feb 15, 2016 at 0:43

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